The overall objective of this study continues to focus on improving our understanding of the functional role of the anterior cruciate ligament. The ultimate goal is to provide information that can be useful in the improvement of the evaluation and treatment of patients with ACL- deficiency. This study will examine the hypothesis that the functional adaptations (identified during the grant period) common to patients with ACL-deficiency, are related to the time past the initial injury and can be associated with clinical changes that progress with time. The second Specific Aim will test the hypothesis that patients who achieve clinical stability following ACL reconstruction will achieve normal function (as quantified by functional biomechanical testing) during activities of daily living and approach normal function in more stressful activities. The hypothesis will be tested by evaluating patients in the gait laboratory with ACL-deficiency on a yearly basis. In addition, patients will be evaluated prior to and following surgical reconstruction of the anterior cruciate ligament with the central third of the patellar tendon. All patients will be evaluated during level walking, stair climbing, jogging, running and cutting, and running to a stop. Functional changes over time will be quantified and correlated with clinical, radiographic, quantitative laxity measurements and muscle strength measurements. The successful completion of this study should provide new information on the function of the anterior cruciate ligament deficient knee as well as the knee following anterior cruciate ligament reconstruction. These results should bring orthopedists one step closer to assessing the clinical implications of the functional biomechanics of the anterior cruciate ligament.